This invenzion relates to a multi-stand rolling mill with a protection or safety device against overloads due in particular to irregularities in the feed of the rolled product.
In multi-stand rolling mills, for example for producing wire rod, the various rolling stands are driven mostly by a single drive motor through a kinematic chain, normally of gear type, each shaft of which drives one rolling stand, the various gears being calculated so as to give the rolls of each stand the correct predetermined speed relative to the rolls of the other stands.
Rolling mills of this type are very compact and also have the advantage that a single motor can drive more than twenty rolling stands.
However in these rolling mills disadvantages arise when irregularities occur in the wire rod feed, such as in the case of a fault in the coiler disposed downstream of the rolling mill for depositing the wire rod in the form of coil turns. In this case, the last rolling stand is no longer able to freely push out the wire rod obstructed downstream, so that this latter becomes axially loaded and begins to deform, bend and crowd up, remaining compressed between the mill rolls and the outlet guide of the rolling stand, to form cobbles.
Under these conditions, the rolls of the stand concerned become subjected to an increasing braking action by the amassed material, which leads not only to an increase in the forces on the drive shaft for the rolls of said stand, but can also damage the gauged surface of the rolls because of the rubbing which occurs between these latter and the rolled material.
To prevent an occurrence of this type, rolling mills have been devised in which protective pin joints of predetermined breakage load are interposed between the kinematic drive chain and each individual rolling stand. When an excessive increase in torque on the roll shaft occurs, as in the case of the formation of cobbles, the protective pin or pins break and the rolls of the stand in question therefore stop. However this stoppage of the stand where the cobble is formed does not completely solve the problem, but causes the phenomenon to be repeated in the stand immediately upstream, the rolls of which have remained in operation and continue to thrust out the rolled product. Thus the protective pin of this stand also breaks, and so on, until all the rolling stands stop due to breakage of their protective pins, when all the stands are blocked with material.
At this point, in order to return the arrested rolling mill to operation, the stands must firstly be completely dismantled to clean them of all the amassed material filling them, and then the broken protective pins must be replaced. It is often necessary to also replace the damaged outlet guides. All these operations lead to considerable wastage of time and correspondingly long shut-downs of the rolling mill, with serious production loss.
To avoid the chain-action breakage of the protective pins, it has also been proposed to stop the rolling mill drive motor or disengage the kinematic chain from the drive motor on formation of a cobble in one of the rolling stands. However such methods are not completely satisfactory, because due to the inertia of the kinematic chain (and motor) cobbles can still form in some of the other rolling stands, so likewise requiring cleaning of the stands.